In recent years, active matrix display apparatuses, such as a liquid crystal display apparatus and AMOLED (Active Matrix Organic Light Emitting Diode), have been developed actively and put in practical use. In particular, large volume and high resolution active matrix display apparatuses have been attracting attention, and the number of pixels used in such an apparatus has been increasing steadily. Further, with the development, a distance between adjoining pixels has been made remarkably narrow, and the narrowing technique has been developed.
Generally, these display apparatuses are manufactured in such a way that a process of forming a thin film on a substrate, such as glass, a photolithography process of forming a pattern on a photoresist, and an etching process of executing etching processing along the photoresist including the formed pattern are repeated by a number of times. With the narrowing of pixels in recent years, etching is needed to be performed with higher processing accuracy. In particular, in a display apparatus with a minimum space less than 4 μm, a process is essential to be performed with dry etching which causes the reduced amount of side etching and has high processing accuracy.
Further, the narrowing of pixels increases the frequency with which even comparatively-small particles conventionally having not caused any problems cause fatal defects. In particular, particles having been generated in a photolithography process cause a defective pattern. Successively, at the following etching process, since etching processing is executed along this defective pattern, a short circuit may be caused between the same layers. However, it is very difficult to remove thoroughly particles generated in the photolithography process.
Then, in order to suppress the occurrence of a short circuit between the same layers, the photolithography process and the etching process have been performed several times for a target film. For example, Japanese Patent Application Laid-Open Publication (JP-A) No. H07-253593 discloses the following processing about forming patterned electrode structures composed of signal wires, a drain portion, and a storage capacitance portion. At the time of the processing, a metal film expected to become the electrodes is formed, and then, the first photolithography and etching processing are executed on the metal film, and thereafter, the second photolithography and etching processing are executed so as to form patterned structures slightly larger than the patterned structures formed at the time of the first photolithography, whereby it is intended to prevent a short circuit in a part of the electrodes, to reduce point defects greatly, to improve the display performance, and to increase the manufacturing yield. In the case of JP-A No. H07-253593, the target film is a drain layer.
Further, JP-A No. H09-230373 discloses the following processing. On a gate electrode formed on a glass substrate, a gate insulating film, an a-Si film, and an n+a-Si film are sequentially laminated, and, patterned structures are formed by the first photolithography. Thereafter, the n+a-Si film, the a-Si film, and the gate insulating film are removed by dry etching, and on the upper portion of the resulting structure, a Cr film is formed. Successively, on the Cr film, patterned structures are formed by photolithography, to form a drain electrode. Then, by utilizing the drain electrode as a mask, the n+a-Si film, the a-Si film, and the gate insulating film which have not been removed by the first photolithography are removed by dry etching. In the case of JP-A H09-230373, the target films are the n+a-Si film, the a-Si film, and the gate insulating film.
Furthermore, JP-A No. 2002-111001 discloses the following processing for a liquid crystal display apparatus in which a top-gate TFT (Thin-Film Transistor) using a polycrystalline silicon thin film in an active layer is employed as a switching element. In the processing, a first photolithography process and an etching process are performed on a metal film on a gate insulating film, and a first photolithography process and a dry etching process are further performed on the remaining metal film with the residue which has not been removed in the first photolithography process and the etching process so as to hold portions corresponding to the form of a gate electrode, the wiring form of a scanning line, the wiring form of an auxiliary capacity line, and the form of a polycrystalline silicon thin film portion and to remove the other part of the metal film. In the case of JP-A No. 2002-111001, the target film is the metal film corresponding to the gate electrode.
In the above-described three articles, dry etching is performed two times in total for the almost entire of the transmissive regions of a display apparatus. Further, in the above-described three articles, the photolithography process and etching process at the second time or the following time are executed for the outside of the TFT. Accordingly, for a portion between a source and a drain on the TFT, the photolithography process and etching process are executed only one time.
On the other hand, in JP-A No. 2005-195891, it is known that color tone of display, i.e., chromaticity changes depending on the thickness of a transparent insulating film. According to FIG. 2 of JP-A No. 2005-195891, chromaticity changes with a period of about 0.2 μm of an insulating film thickness.
However, in the structure or manufacturing method in JP-A Nos. H07-253593, H09-230373 and 2002-111001, although defects due to a short circuit between the same layers of a conductive layer may be prevented, the following two problems take place.
The first problem is a point that the display quality of a display apparatus, in particular, chromaticity at the time of displaying white changes, and a point that a difference occurs in a level of the change depending on a position in a display area or a position in a substrate. The reasons are that an amount of change in the thickness of an insulating film in each transmissive region becomes large due to dry etching performed two times in total for the almost entire of the transmissive regions of a display apparatus, and that a difference in the amount of change of the thickness depending on a position in the substrate surface becomes large in association with it. In particular, in RIE (Reactive Ion Etching) having been used well as dry etching with less side-etched portions, since ions are accelerated at the time of entering a substrate, not only a conductive layer being a processing target, but also, its undercoat insulating film is etched, which necessarily causes excavation into the undercoat insulating film. Accordingly, the thickness of the undercoat insulating film may change. Further, since dispersion exists in an etching rate within a substrate surface, a difference in an amount of change of the thickness of the undercoat insulating film becomes larger depending on a position in the substrate surface.
Here, a change of the thickness of an undercoat insulating film is estimated in the following process model. For example, if an aluminum film (Al film) (with a thickness of 400 nm) on a silicon oxide film (SiO2 film) with a thickness of 440 nm±44 nm is processed by dry etching with an etching rate dispersion of ±15%, a selection ratio of 5, and an over etching ratio of 50%, the SiO2 film is excavated by 40 nm (=400 nm×0.5/5)±6 nm, the thickness of the SiO2 film becomes thin to 400 nm±50 nm, and a range of the film thickness dispersion is expanded. In addition, if the second photolithography and dry etching process are performed in order to prevent a short circuit between the same layers of an Al film, an amount of excavation into the SiO2 film further increases. This is because, in order to separate securely a portion having been not separated at the time of the first dry etching by the second dry etching, it is necessary to perform the second dry etching for the almost same time period as the first dry etching. Therefore, the surface of the SiO2 film of a portion having been normally separated at the time of the first dry etching is etched from the start time of the second dry etching, and then, an amount of excavation into the SiO2 film further increases. In the above example, by the second dry etching, the SiO2 film is further excavated by 120 nm (=400 nm×(1+0.5)/5)±18 nm, that is, excavated by 160 nm±24 nm in total. This is a value almost equivalent to the period of the insulating film according to FIG. 2 of JP-A No. 2005-195891, and the chromaticity of a display apparatus using this insulating film is made to change greatly. In the above example, it would be estimated that the SiO2 film becomes thin finally up to 280 nm±68 nm and a range of film thickness dispersion is made to expand.
With regard to chromaticity when white is displayed by a display apparatus which uses this silicon oxide film (SiO2 film) for transmissive regions of an active matrix substrate, an optical simulation has been used to analyze how the chromaticity changes due to a change of the thickness of the SiO2 film, and FIG. 1 illustrates an example of the result of the analysis. The film constitution of the transmissive regions was made nine layers based on the supposition of a top-gate TFT having used a polycrystalline silicon thin film for an active layer as shown in TABLE 1, and was defined such that the top and bottom of them are sandwiched by air layer. These nine layers was configured such that on a glass substrate (Layer 1), undercoat films (Layer 3, Layer 4) composed of silicon nitride (SiNx) and silicon oxide (SiO2) were disposed, then, thereon, a gate insulating film (Layer 5) composed of silicon oxide (SiO2), a first interlayer insulating film (layer 6) composed of silicon oxide (SiO2), a second interlayer insulating film (Layer 7) composed of silicon nitride (SiNx), further, an organic layer (Layer 8), and an ITO (Indium Tin Oxide) film (Layer 9) were laminated. Here, in order to adjust the absorption coefficient of the glass substrate, Layer 2 of BK7 was inserted. Among them, by changing the thickness of the first interlayer insulating film (Layer 6) in a range of from 160 to 640 nm, the optical simulation was performed so as to obtain a change of the chromaticity. In its calculation method, the transmittance when light rays of red (R), green (G), and blue (B) being three primary colors of light pass through a film constituted as shown in TABLE 1 was obtained while changing the thickness of the Layer 6, and then, from these, the chromaticity coordinates (the xyY color coordinate system) x and y at the time of displaying white were obtained, and plotted as illustrated in FIG. 1. As illustrated in FIG. 1, in a range of 160 nm to 640 nm of the thicknesses of the Layer 6, the chromaticity x has changed in a range of 0.30 to 0.35, and the chromaticity y has changed in a range of 0.30 to 0.37. Further, the fluctuation period of the chromaticity y was about 160 nm. Further, at the lower portion of FIG. 1, three thickness dispersion ranges A to C are indicated with respective arrow heads based on the example of the above-mentioned process model. The range A corresponds to the case right after the film formation and represents a rage of 440 nm±44 nm (a film thickness fluctuation width of 88 nm), the range B corresponds to the case after the first dry etching and represents a rage of 400 nm±50 nm (a film thickness fluctuation width of 100 nm), and the range C corresponds to the case after the second dry etching and represents a rage of 280 nm±68 nm (a film thickness fluctuation width of 136 nm). Next, comparison was made the influence given to the fluctuation width of the chromaticity by each of the ranges A to C. In this comparison, the chromaticity y which had a specifically large fluctuation width was made a comparison target.
TABLE 1RefractionLayerindexFilm thicknessRemarksAir1Glass substrate1.555mm2BK71.5250nm*13SiNx1.8650nmUndercoat film4SiO21.5100nmUndercoat film5SiO21.5120nmGate insulating film6SiO21.5160 nm toFirst interlayer640 nminsulating film7SiNx1.86400nmSecond interlayerinsulating film8Organic film1.51000nm9ITO240nmAir*1: Inserted for correcting the absorption coefficient of the substrate
First, in the range A immediately after the film formation of the first interlayer insulating film, the chromaticity x has changed in a range of 0.32 to 0.33, and the chromaticity y has changed in a range of 0.30 to 0.37. At this time, the range of fluctuation of the chromaticity y was 0.07.
Next, in the range B after the first dry etching, the chromaticity x has changed in a range of 0.32 to 0.33, and the chromaticity y has changed in a range of 0.30 to 0.35. At this time, the range of fluctuation of the chromaticity y was 0.05. In the range B, as compared with Range A, in spite of the enlargement of the film thickness fluctuation width, the fluctuation width of the chromaticity y has reduced.
The reason is considered that, in contrast to the range A which corresponded to a section in which the chromaticity y has changed from the maximum value to the minimum value, the range B was a section which had the minimum value of the chromaticity y at its almost central portion. In this way, depending on the thickness of the first interlayer insulating film, the fluctuation width of the chromaticity y can be made small. This was because the film thickness fluctuation width was as small as about ⅔ of a fluctuation period of 160 nm of the chromaticity y.
On the other hand, in the range C after the second dry etching, the chromaticity x has changed in a rage of 0.30 to 0.33, and the chromaticity y has changed in a rage of 0.30 to 0.37. At this time, the fluctuation width of the chromaticity y was 0.07. This range C was a section in which the chromaticity y has changed from the maximum value to the minimum value. Further, since the film thickness fluctuation width expanded to about ¾ of a fluctuation period of 160 nm of the chromaticity y, even if the first interlayer insulating film was formed with any size of the film thickness, the range C might be made to become a section in which the chromaticity y might change almost from the maximum value to the minimum value. Therefore, it is difficult to suppress the fluctuation width of the chromaticity y to be small.
As mentioned above, in accordance with the respective structures and manufacturing methods of JP-A Nos. H07-253593, H09-230373 and 2002-111001, if the second dry etching is performed not only for non-transmissive regions, but also for the almost entire region of the transmissive regions, excavation into the undercoat insulating film increases, and the thickness of the undercoat insulating film becomes thinner. Further, dispersion in film thickness is made to expand. Accordingly, the display quality of a display apparatus, in particular, the chromaticity at the time of displaying white is made to change. Further, depending on a position in a display area, or a position in a substrate, a difference is caused on its level.
In addition to that, if taking a micro loading effect into consideration, an amount of excavation into the undercoat insulating film of transmissive regions tends to increase more. That is based on the following reasons. Generally, as compared with the transmissive region, in the non-transmissive regions, since the patterns, such as wiring, are dense, the opening portion of the resist is small as compared with the transmissive regions. Accordingly, due to a micro loading effect, as compared with the transmissive regions, in the non-transmissive regions, an etching rate tends to lower. Therefore, if the etching time is set up in conformity to the non-transmissive regions, the undercoat insulating film of the transmissive regions is etched more, and an amount of excavation tends to increase more.
Then, in order to control the chromaticity of a display apparatus, it is necessary to control precisely the thickness of an undercoat insulating film of transmissive regions. However, dispersion necessarily exists in film formation facilities and dry etching facilities and such dispersion becomes a factor of fluctuation. Therefore, a structure and a manufacturing method configured to reduce the factor of fluctuation as small as possible are required.
The second problem is a point that, in each of JP-A Nos. H07-253593, H09-230373 and 2002-111001, since the photolithography process and etching process are executed multiple times for the outside of the TFT and are executed only one time for the TFT, there is no effect for a short circuit between the source and drain electrodes of the TFT. Further, as the countermeasure for the above point, if a pattern to separate between the source and drain of the TFT is simply added at the second photolithography process, a LDD resistance changes in a TFT having a LDD (Lightly Doped Drain) structure, and it becomes a factor to change the TFT characteristic. The reason is that if an insulating film on an LDD portion is etched two times or more by dry etching, since the film thickness and surface state of the insulating film change, the fixed electric charge and trap level in the insulating film on the LDD portion are made to change, and then, an effective LDD layer resistance is made to change. If the LDD layer resistance changes, since an ON current and OFF current of the TFT change, the display quality of a panel is influenced greatly.
FIG. 2A illustrates the TFT characteristic in the case where the second photolithography and dry etching to separate source and drain electrodes were executed in a TFT having a LDD structure in a P-channel, and FIG. 2B illustrates the TFT characteristic in the case where the second photolithography and dry etching were not executed. In the characteristic in FIGS. 2A and 2B, a leak current between source and drain was measured by replacing the source and drain electrodes while irradiating light on a backlight. In FIGS. 2A and 2B, the gate voltage is given as Vg=8V. As illustrated in FIGS. 2A and 2B, it turns out that if the second photolithography and dry etching to separate the source and drain electrodes are executed on the LDD, the symmetric property of the leak current changes. This causes display abnormalities, such as crosstalk and flicker, in the display of a panel.
In addition to the above two points, there exists a point to be worried. That is a point that if etching is performed two times or more for a conductive film by dry etching in order to prevent defects due to a short circuit between the same layers of the conductive film, an amount of excavation into a undercoat insulating film increases, and a difference in level in combination of an amount of excavation into a undercoat insulating film and the thickness of the conductive film is made to increase. In the case where an insulating film is further formed on the conducting film, this affects the coatability of them. That is, in the case of a PECVD (Plasma Enhanced Chemical Vapor Deposition) film, due to the deterioration of coverage is, and in the case of a coating film, due to the abnormalities of fluidity in the difference in level at the time of coating and spreading by a spin coater etc., streak-like coating unevenness, so-called striation may be caused. In this connection, the larger difference in level tends to cause the deterioration of striation. The deterioration of coverage may cause the deterioration of the yield and reliability of a display apparatus, and the deterioration of striation may cause deterioration of display qualities, such as display unevenness. The present invention seeks to solve the problem.